The subject matter disclosed herein relates generally to magnetic resonance imaging (MRI) systems, and more particularly to a method and apparatus for imaging a patient using local shim coils.
MRI or Nuclear Magnetic Resonance (NMR) imaging generally provides for the spatial discrimination of resonant interactions between Radio Frequency (RF) waves and nuclei in a magnetic field. Specifically, MRI utilizes hydrogen nuclear spins of the water molecules in the human body, which are polarized by a strong, uniform, static magnetic field of a magnet. This magnetic field is commonly referred to as B0 or the main magnetic field. When a substance, such as human tissue, is subjected to the main magnetic field, the individual magnetic moments of the spins in the tissue attempt to align with the main magnetic field. When excited by an RF wave, the spins precess about the main magnetic field at a characteristic Larmor frequency. A signal is emitted by the excited spins and processed to form an image.
However, in operation, variations may occur in the strength of the main magnetic field. Such variations in the main magnetic field may affect the acquired images. For example, when a conventional MRI system generates a main magnetic field of, for example, 3 Tesla, the variation of the main magnetic field due to the magnetic susceptibility of human body may be on the order of approximately 100 Hz. Therefore, when the conventional MRI system is utilized to perform, for example, breast imaging, a 100 Hz variation may adversely affect fat saturation, breast MR spectroscopy, and/or Echo Planar Imaging (EPI) readout. In particular, EPI pixel shift and distortion caused by the main magnetic field variation may reduce spatial resolution and/or decrease the Signal-to-Noise (SNR) of diffusion-weighted imaging.